drop support for node 6 and 8

BREAKING CHANGE: Dropping support for node 6 and 8.  It'll probably
still work on those versions, but they are no longer supported or
tested, since npm v7 is moving away from them.

.travis.yml

@@ -1,8 +1,5 @@
 language: node_js
 node_js:
-  - "12"
-  - "10"
-  - "8"
-  - "6"
-sudo: false
-script: "npm test"
+  - node
+  - 12
+  - 10

package.json

@@ -40,5 +40,8 @@
   "bugs": {
     "url": "https://github.com/npm/npm-package-arg/issues"
   },
-  "homepage": "https://github.com/npm/npm-package-arg"
+  "homepage": "https://github.com/npm/npm-package-arg",
+  "engines": {
+    "node": ">=10"
+  }
 }